In a tube-type color camera the effective exposure time is 1/30th of a second, comprised of two interlaced fields of 1/60th of a second each. One of the drawbacks in tube-type cameras is that they exhibit low resolution when picking up fast moving objects. In the past an optical shutter has been evaluated in such cameras to shorten the exposure time, however, the results have not been impressive due to the lag characteristics of tube-type sensors.
A solid-state camera has zero lag and, therefore, is a good sensor for picking up fast moving objects. With such a zero lag device it is now feasible to use optical shuttering to enhance the dynamic resolution of the camera. In a frame transfer (also known as a field-transfer) charge-coupled device it is generally considered to be necessary that the imager be optically shuttered during the interval (vertical blanking) in which the charge accumulated in the imaging area of the device is transferred to the storage area of the device. Failure to shutter during this period causes vertical image smear, most noticeable when picking up scenes containing highlights. In a frame transfer CCD having 403 horizontal elements the effective exposure time is 1/60th of a second due to the method of signal readout. Interposing a mechanical shutter (rotating wheel) between the main camera lens and the prism system eliminated frame transfer smear and incidently achieves a limited amount of optical shuttering.
In U.S. patent application Ser. No. 330,033 filed on Dec. 14, 1981, in the name of R. A. Dischert a solid-state camera is disclosed which includes a shutter arrangement for controlling the exposure time of the camera. In accordance with one aspect of the Dischert application, two similar shuttering devices are arranged in series such that the combined effective width of the shutter blades may be adjusted to improve the resolution of the moving images. Two shutters are arranged coaxially on shafts which are long enough to protrude through the camera housing so they are accessible to allow relative rotation therebetween. With this arrangement, the light passage between the blades of the shutter system can be reduced to a slit to provide "true" stop motion of an image during action sequences.
In accordance with the present invention a shuttering system is provided which permits automatic exposure time control of a solid-state camera. For example, two similar shutters are arranged coaxially. One shutter motor is locked to vertical sync while the other shutter motor which is also locked to vertical sync is provided with a variable phasing control. Variation of the relative phase of the two shutters changes the effective exposure time.
A variable shutter exposure control permits the camera operator to select different exposure techniques for artistic or special-effect purposes. Essentially all television cameras have an automatic iris control. Control of the iris is normally based on a measurement of the highest signal (in the red, green or blue channel) in the central area of the picture. The iris drive motor is servoed in a manner such that constant peak signal level is maintained at the camera output. According to this method the exposure time control of the shutter is varied manually and constant output signal is maintained by the automatic iris servo. In an alternative method of operating, the camera system could be run so that the iris is positioned in a fixed position and then servo the exposure time of the shutter to achieve constant camera output signal. Such a dual system is analagous to that used in modern single lens reflex film cameras. Such systems are referred to as "aperture preferred" or "exposure preferred".